With OSS you get more people working on a project that actually care. A proprietary business project prioritizes making money over actually creating a good product everyone loves.

You're right that this is not a perfect solution. All software has bugs and all software may have malicious back doors. I just find it much easier to trust the development that happens in the open with community involvement than the development that happens in secret where I have absolutely no way see what's going on.

If you had an inkling that someone was trying to poison you, would you rather eat the food you watched be prepared or the food that was prepared in secret? Both dishes might be poisoned, but it's reasonable to prefer the one you were able to examine.